Cable insulation



ocr. 26 192s. l 1,604,334

Y R. W. ATKINSON CABLE INSULATION 4 Filed May 12, 1925 MWMMW 76mm;

` medial and longitudinal section of a cable Patented Oct. 26, #1.926.

UNITED s'rii'rE-sy PATENT OFFICE.

RALPH W. ATKINSON, OF PERTH NEW JERSEY, ASSIGNOR TO STANDARD UN- DERGROUND CABLE COMPANY, OF PITTSBURGH, PENNSYLVANIA, A CORPORATION OF PENNSYLVANIA.

CABLE INSULATION.

Application filed May 12, 1923. Seria1 No. 638,572.

My invention relates to the construction of electrical cables, and particularly to the envelope o f wrapped-on insulation which dimensions, and without those modifications in shape of the component parts ofia cable incident to building, a cable so far built up as to illustrate my invention. Fig. I is a view partly in -side elevation, partly inl built according to my invention; Figs. vII and III are similar sectional views, drawn to larger scale, showing the cable insulation alonev in fragmentary manner, and more fully illustrating the invention. The planes of section ofFigs. II and'lII are at an angle one to the other, 'and wo-uld so appear if plotted on a drawing of the cable in cross section. The body .of the cable conductor itself is omitted from Figs. II and III, to make the showing simpler. l

T he insulation of an electrical cable commonly includes tape, formed perhaps of paper, or of. fabric, or even ofV rubber, wrapped helically about the central core. The helix may be laid with the edges of successive turns spaced apart, or closely contiguous, or overlapping. ',Ihe insulation value of tlie laminate structure so built up is greatest, of course, where thej'tape lies in continuity, layer upon layer; and this maximum insulation value is diminished at the seam which is formed. at or between the adjacent edges of the successive turns.

y There spaces inevitably are found. And

this is true whether 'the helix be laid in one or'another of. the alternative modes mentioned; and it is true, however small the spaces may be. which are formed at the seam, and with whatever substance these spaces be iilled--whether with a saturating and filling compound applied in liquid forni,

or with air or other gas under high or low pressure. i For certain manufacturing reasons, it 1s not always desirable to wrap the tapes of the entire envelope of insulation, all in the same direction. That is to say, in the usual cable-building operation, a number of tapes will be laid on in one direction, and, succeeding them, an additional number will be laid in reverse direction, and there may be several such reversals in the direction of lay, in the building up of the whole insulating envelope.. vWhen layers wrapped on inthe same direction are superposed, the seams be` tween the' turns do not directly overlie one another; the seam of a superior layer lies uponthe continuous web of the tape of the inferior layer. the lay7 is reversed the seams in the But where the direction ofl superior layer will cross the seams in the inferior, at' recurrent points the condition of diminished insulating value at the A'seam due to the superposing of weak portions is more' serious than where the gaps occur separately. 1

The choice of the thicknessof tape is the result of a compromise between desirable properties which are secured on the one hand by thick tape and other desirable properties which' `are secured on the lother hand by thin tape. Theg'oing to an extreme in eitherv direction, that is althick tape or a thin tape, results. in a substantial loss in the eii'iciency of the insulation `as a whole. `L

A primary advantage of thick tape 'over thin lies in its greater strength -to lresist tearing. Thin tapemay be 'made to have high tensilestrength, Sbu't.V even s'o,' 'its strength to resist' tearing is, relative to its thickness, low. -Tearing strength is strength to Iwithstand' jtwisting" strains;' tensile strength is strength' to resist strains vexerted .in lthe plane of them'aterial." In'l'prac'tice i when the tape of arable .has sufferedxinjury, the yinjury will generally be found to be 'a'ttributable, not to its lack of tensile strength, but rather to its liability to tearing. It may have been torn while being applied or it may have been torn under stress incident to the bending of the finished cable. Even if the thinner tape were proportionately to its thickness as strong to resist tearing as is the thicker tape, still thinner tape would be at a disadvantage, for the stresses of mani ulation to which a cable necessarily is su ject are vthe same, whether the tape be thin or thick; and when, added to that., it appears that thin tape is disproportionately weak,

the advantage of Athick tape will be more clearly recognized.

A primary disadvantage of the thick tape as compared with the thin is in the thickness of this seam space. The cable designer then must in particular cases make choice between two evils; thick tape for vother reasons pref-A erable must when applied give vrelatiyelyl large seam spaces; thin tape, which gives smaller seam spaces, is for other reasons. in. erior; and the designer must make a compromise. My invention reduces this diii culty, and allowing him to use for' the most part the preferred thick tape, relieves him of the difficulty of large seam spaces:

My invention lies in this, I use iny t'hose wrappings which are superposed in spirals of the same direction relatively thick tapes, and in those wrappings Where superposition is in reversed direction, relatively thintapes; 'Ihat is to say, I retain for themost part thick tape with its peculiar advantage; but,- at the point where the tendency to defect is enlarged, I use thin tape, and in so -doing counteract that tendency. Stated in. other words, in a laminate insulation built up by applying tapes in helical wrapping, when it becomes desirable to reverse the direction of wrapping, the last'tape of the underlying layer and theQ first tape..of the overlying. layer are of diminished thickness. Thus the otherwise' enlarged' spaces incident to reversal in direction of wrapping are kept small. .l

I find that in thepractice of my invention the dilemma stated" above .is substantially avoided; I retain the advantage. of thick tape', without appreciableloss, and I avoid the evil of large seam spaces.

The invention isapplicable, whatever be the particular modefof laying the helical' Wrapping.

Referring to the drawings, the conduction core of the cable'is indicated atI 1, an underlying layer ofwrapped-on tape is indicated. at.2 and an overlying layer at`3. These layers are wrapped on iiropposite directions.

The outermost wrap v4 of the underlyinglayer andtheinnermost wrap 5 of the over-j lying layery are shown4 to be -of tape which,

as eomparedwith the remainder of the struc-,

' uniform throughout the ture, l here used arbitrarily to connote a portion ofthe whole .envelopeof insulatiom-a portion built up of tape applied in'a succession of turns, the directionfof wrapping being y portion so particularly 'dened" as a'layer. The term layer is so employed in theen'si'i'ing claim;

The drawings show the inyention applied is relatively thin. The term layer is to tapel wrapped'onin open helix with the edges of successive turns spaced apart. It is manifest that the same pointimust beinvolved,`.however closely the edges/'may ap# proach oneil another, and even in case the edges overlap. Comparison of Figs. II and III will indicate how atrecurrent intervals the relatively small spaces in layers4 land 5 are superposedzand increased-.sinsize But the spaces in these particularlayers are rela-- tively small, and when superposed the;ag-. gregate-size is still not too great. It .will of course be remembered, butthel observation should herebe repeated, thatthe showing,- aiforded in these drawings is diagrammatic, In the buildingiof a vcablethe number of; superposed, wraps in each direction may manifestly 'be increased indefinitely, beyond l,the f ew`t'urns here shown diagrammatically;

For the sake ofsimplicity of showing, theA number of layers-andthe numberrof reversalsv gf wrapping are intentionally; reduced to a The invention manifestly is applicable to Wrapped-on insulation` gener'ally,. and in'this. respect the showing of yit in` application to conductor is exemplary merely. Y,

I claim as my invention l:

An electrical. cable including a conductor and a surrounding envelopeof helically wrapped tape, the tape envelope` being made up of superposed layers. and the tap within each layer being laid iiruniformlyl directed spirals and the tape of successive layers be-L- ing laid in oppositely directed spirals, the'l last tape ofK an inferior, layer and the rst` tape of a superior layer being fless in thick` ness than the tapes beneath and above.

In testimony whereof I have hereunto set my hand.

RALPH. W. ATKINSON. v 

